Factors Affecting the Choice of Reactor

To move through the sizing process we consider various reactants, top RHS of Fig. 6.1 and select possible reaction routes from the chemistry, understand competing and unwanted side reactions, select the phases and decide if a catalyst should be used. As illustrated on the RHS of Fig. 6.1, having a catalyst introduces questions of selectivity, activity, size, porosity, life, contaminants and poisons that interfere with the catalysf s function and temperature limitations. Our choice of reaction route also sets the heat release from the reaction highly exothermic or 

Rules of Thumb in Engineering Practice. Donald R. Woods Copyright 2007 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-31220-7 

Criteria seletivity, yield cost, thermal control, or flexibility 

Selectivity cheap easy multiple reactions producing byproducts  

Heat of reaction Reversible Order of irre- Inlet temperature Control of temperature  

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We first present further examples of the types of reactions involved in two main classifications, and then a preliminary discussion of various types of reactors used. Following an examination of some factors affecting the choice of reactor, we develop design equations for some reactor types, and illustrate their use with examples. The chapter concludes with a brief introduction to trickle-bed reactors for three-phase gas-liquid-solid (catalyst) reactions. 

Factors Affecting the Choice of Reactor 189 Table 6.3 Parameters related to hazard and reactor safety. 

Table 6.6 Continued. 6.7 Factors Affecting the Choice of Reactor 193 ... 

The choice of reactor temperature depends on many factors. Generally, the higher the rate of reaction, the smaller the reactor volume. Practical upper limits are set by safety considerations, materials-of-construction limitations, or maximum operating temperature for the catalyst. Whether the reaction system involves single or multiple reactions, and whether the reactions are reversible, also affects the choice of reactor temperature, as we shall now discuss. 

In fine chemistry, mathematical models are scarce yet. However, even gross kinetics provides a lot of information on the influence of the mode of operation on seleetivity. In general, semi-quantitative criteria are used in preliminary reactor selection. They are mainly based mainly on operational characteristics, experience, and a rough economic estimation. Factors affecting the choice of the reactor and mode of operation are listed in Table 5.4-42. 

It is not proposed to deal with each Item used In the reactor but to discuss the factors affecting the choice of materials in a broader perspective as the choice of the major items, and of the water chemistry regime interact upon one another, and a decision to fix on one may limit the freedom of choice for the others. 

When combining the separator and the reactor functions into one compact physical unit, factors related to catalysis need to be considered in addition to those related to selective separation discussed in previous chapters. The selection of catalyst material, dispersion and heat treatment and the strategic placement of catalyst in the membrane reactor all can have profound impacts on the reactor performance. The choice of membrane material and its microstructure may also affect the catalytic aspects of the membrane reactor. Furthermore, when imparting catalytic activity to inorganic membranes, it is important to understand any effects the underlying treatments may have on the permeability and permselectivity of the membranes. 

The flow also depends on the reactor shape. The cylindrical tubular reactor is the most used but there are other shapes as conical and cylindrical trunk with transversal flow, which change the velocity profile affecting the flow as shown in Figure 14.1. The choice of the type of reactor depends on the flow and other factors ... 

The choice of the moderator material for a central-station powerplant is generally based on the economics involved. Obviously, many factors other than the cost per unit weight or volume, per se, enter into the economics. The neutron slowing-down capability of the material has an important effect on the size of the reactor core and, therefore, the capital cost of the plant, because of the investment in moderator, pressure vessel, shielding, etc. Containment requirements for the moderator (particularly liquid moderators) can affect both the capital cost of the plant and the fuel cycle economics, the latter because of possible neutron losses. Integrity and stability of the moderator material can, of course, have important implications on other aspects of the reactor design. The neutron absorption behavior of the moderator itself affects the potential conversion ratio of the reactor and, therefore, the fuel cycle economics of the reactor. The properties of the more important moderators and the implications of these properties on the choice and performance characteristics of gas-cooled reactors will be reviewed in this section. 

Factorial design can be applied to any number of factors and levels. Frequently, in exploratory work, only two levels of each factor are chosen for the factorial design. Two factors and two levels constitute a 22 factorial design and permit setting up a 2 x 2 analysis-of-variance table with one factor on columns and another on rows. By determining the row effect and the column effect, it is possible to determine, for example in a reactor, whether a variation in temperature or pressure affects the reaction yield. A choice of substantially different values of temperature and pressure would be desirable to ensure conclusions which would be based on a wide range of factors. 

Table HI lists the neutron absoiption cross sections for many of the metals described above, as well as their cross section relative to the typical reactor material, zirconium. Materials with a very large cross section relative to zirconium would result in a reduction in the thermal utilization factor f and hence a reduction in Nff. Consequently, Ta, W, V, Mo and Ni based alloys would be impractical choices for a reactor core. From this literature survey, it appears that Fecralloy would provide the greatest promise as a containment material for liquid lead. In addition Tantiron may be an alternate choice. More extensive studies on the applicability of inhibitors such as Ti should be undertaken to determine their affect on the corrosion resistance of these materials. 

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